There are many instances in which it is desirable to inject chemicals of various types into fluids (gas and liquids) flowing in pipelines. One such example is in the area of natural gas pipelines. In addition to such substances as corrosion inhibitors and alcohol to inhibit freezing, odorants are commonly injected into natural gas pipelines. Natural gas is colorless and odorless. Odorant is injected into natural gas in order to provide a warning smell for workers and consumers. Commonly used odorants include tertiary butyl mercaptan (TBM). Such odorants are typically injected in relatively small volumes normally ranging from about 0.5 to 1.0 lbs/mmscf.
The odorants are typically provided in liquid form and are typically added to the gas at a location where distribution gas is taken from a main gas pipeline and provided to a distribution pipeline. In such circumstances, the gas pressure may be stepped down through a regulator from, for example, 600 psi or more, to a lower pressure in the range of 100 psi or less. The odorants can also be added to the main transmission pipeline in some situations.
As can be seen above, the odorants which are added to natural gas are extremely concentrated. Odorants such as TBM and other blends are mildly corrosive and are also very noxious. If the injecting odorant into distribution pipelines is not performed accurately, lives are sometimes endangered. It would be possible for a homeowner to have a gas leak without it being realized until an explosion had resulted if the proper amount of odorant was not present. Also, if a leak of odorant occurs at an injection site, people in the surrounding area may assume that a gas leak has occurred even though natural gas may not be physically present. This may result in areas being evacuated and commerce being interrupted. Contrarily, if such mistakes become common, people in the surrounding area may become desensitized to the smell of a potential gas leak and may fail to report legitimate leaks.
Two techniques are commonly used for providing odorization to natural gas in distribution pipelines. One technique involves bypassing a small amount of natural gas at a slightly higher pressure than the pressure of the main distribution pipeline, through a tank containing liquid odorant. This bypass gas absorbs relatively high concentrations of odorant while it is in the tank. This heavily odorized bypass gas is then placed back into the main pipeline. The odorant, now volatilized, diffuses throughout the pipeline. However, there are a number of disadvantages associated with the bypass system for odorizing pipelines. One disadvantage of the bypass system is the fact that the bypass gas picks up large and inconsistent amounts of odorant from the liquid in the storage tank and becomes completely saturated with odorant gas. As a result, it is necessary to carefully monitor the small amounts of bypass gas which are used. Also, natural gas streams typically have contaminates such as compressor oils or condensates which can fall out into the odorant vessel in bypass systems. These contaminates create a layer that reduces the contact area between the liquid and the bypass stream. This necessarily degrades the absorption rate of the stream failing to accurately measure and control the amount of odorant being added to the stream. This absorption amount can change as condensates and other contaminates fall out and change the absorption boundary layer.
Another technique involves the injection of liquid odorant directly into the pipeline through the use of a high-pressure injection pump. High-volume odorizers have depended on a traditional positive-displacement pump or solenoid valve to deliver discrete doses of odorant to natural gas or liquid propane gas (LPG) streams for the purpose of bringing these streams to safe perception levels. However, injecting discrete doses in this manner results in higher pressure drops due to the higher piston speed. The higher the piston speed, the more likely the odorant will vaporize and the more likely entrainment of gas. Such vapor lock is detrimental to the performance and accuracy of odorant injection systems. These methods can leave dangerous dead time between doses. Because odorant is extremely volatile, drops injected to the pipeline immediately disperse and spread throughout the gas in the pipeline. In this way, within a few seconds, the drops of liquid odorant are dispersed in gaseous form.